1. Field of the Invention
The invention relates to integrated digestible wastes (sewage and otherwise) and waste polar fats/oils/greases/waxes (FOG) treatment methods, systems and facilities including anaerobic digesters and steady-state generation of methane.
2. Description of the Prior Art
Excessive concentrations of FOG (fats, oils, waxes and greases) are a major problem in wastewater/sewage collection and treatment systems. There are two types of FOG. The first type, polar FOG originates from animals or vegetable (foods). Because of the prevalence of food service and processing enterprises in populated environs, polar FOG is responsible for a large percentage of sewer system failures and overflows. In particular, polar FOG, if not intercepted congeals on and sticks to piping and fixtures in wastewater systems, and, as well, to other debris flowing in the waste stream creating plugs and causing functional failures of the sewer system components.
The second type, non-polar FOG, is from petroleum or mineral origins, i.e., is petroleum-based oils, waxes, and greases. This type of FOG is typically detrimental to wastewater treatment systems/processes and, in particular, to the biologic phase of waste treatment process inhibiting (poisoning) microorganisms that breakdown or digest the wastes. Introduction of non-polar FOG into a sewage treatment system is generally prohibited by law and penalized when possible.
Publicly owned sewage treatment systems typically, by statutes, ordinances, and/or regulations, require food service and processing enterprises discharging waste in to public systems to have, and regularly maintain grease traps and interceptors to prevent introduction of generated polar FOG into treatment systems. Grease traps and interceptors are baffled tanks or basins that functionally rely on the immiscibility and the different densities of wastewater and FOG. Wastewater with FOG and other materials are input near the tank top and pools on one side of the baffle(s). The wastewater flows the beneath the baffles trapping less dense FOG floating, cooling and congealing on the water surface behind the baffle. The more dense and FOG'ed (coated) solid materials input with the wastewater settle to the tank bottom. Wastewater exits the tank near the top on the opposite side of the baffle(s) for the most part, sans low density FOG and other high-density materials. However, grease traps and interceptors have limited capacities. The accumulated, floating and congealed FOG and heavier coated materials settled on the tank bottom must be regularly removed, otherwise the trap/interceptors can plug up. Or more seriously, as the traps/interceptors approach capacity, input wastewater tends to entrain both FOG and other materials as it streams through the FOG/solids filled trap/interceptor tank inducing failures downstream in the sewer system.
Best management practices taught by publicly owned wastewater treatment operations mandate regular clean outs of both grease traps and interceptors preferably by professional FOG haulers and recyclers licensed for handling and properly disposing of ‘BROWN’ FOG, i.e., polar FOG contaminated with raw sewage and solids that typically collect in grease traps and interceptors. The simplest and probably most economical mode of cleaning out grease traps and interceptors is to isolate the trap/interceptor in the plumbing system, and then to pump or ‘vacuum’ the entire contents of the trap or interceptor tank/basin into a truck or trailer tank. Caked grease/fat is then steam or pressure-cleaned from the interior tank/basin walls with the wash accumulate being vacuumed into the truck or trailer tank. The so tanked ‘BROWN’ FOG evacuate from grease traps/interceptors is an unholy, difficult to handle, smelly, watery, sticky mess.
Properly disposing of such tanked ‘BROWN’ FOG evacuate is a problem precisely because it is contaminated, contains rocks, glass, tableware, both broken and not, bones and other items people thoughtlessly, or purposely toss/flush down drains at public and commercial establishments. Historically, such ‘BROWN’ FOG evacuate has been deemed ‘toxic’ and is required to be disposed of in landfills appropriately isolated from aquifers and surface drainages. Existing appropriate disposal sites are filling up, and new sites are difficult to find, and once found, economically expensive to establish and maintain. Finally, after a landfill site reaches capacity, its possible uses are limited far into the foreseeable future.
It is well recognized in wastewater management fields that ‘BROWN’ FOG also comprises a source of feedstock suitable for digestion, with the benefits of biogas production including methane for electrical power generation and heating. For example, the South Bayside System Authority (SBSA) located in Redwood City, Calif. has been accepting ‘BROWN’ FOG evacuate from 13 or so permitted haulers since the early 1990's at rates of 1500 to 3000 gallons per day, that after removal of indigestible solids (rocks, glass & tableware) produce approximately 20 cubic feet of digester gas (60% methane) per gallon of greases when introduced into a single mesophilic anaerobic digester. Problems experienced at the SBSA facility primarily relate to handling of the ‘BROWN’ FOG evacuate, and to spiking of biogas production with each cleaned ‘BROWN’ FOG evacuate loaded into the digester. The Wastewater Division of the City of Oxnard, Calif. (OWD) actually provides a municipal grease trap/interceptor cleanout service with personnel and vacuum trucks for local food servicing and processing enterprises. The collected ‘BROWN’ FOG evacuate is input via grease feed and horizontal chopper pumps to one of three 110 foot anaerobic digesters where mixing is enhanced using gas draft tubes. Again, the problems experienced at the OWD treatment plant, akin to those at SBSA, relate to material handling (clogs) and spiking biogas production when the ‘BROWN’ FOG evacuate is offloaded to the digester.